13.2 The central nervous system

The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person’s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord.

The cerebrum

The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the cerebrum    ( [link] ). The wrinkled portion is the cerebral cortex    , and the rest of the structure is beneath that outer covering. There is a large separation between the two sides of the cerebrum called the longitudinal fissure    . It separates the cerebrum into two distinct halves, a right and left cerebral hemisphere    . Deep within the cerebrum, the white matter of the corpus callosum    provides the major pathway for communication between the two hemispheres of the cerebral cortex.

The cerebrum

Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The complexity of the cerebrum is different across vertebrate species. The cerebrum of the most primitive vertebrates is not much more than the connection for the sense of smell. In mammals, the cerebrum comprises the outer gray matter that is the cortex (from the Latin word meaning “bark of a tree”) and several deep nuclei that belong to three important functional groups. The basal nuclei    are responsible for cognitive processing, the most important function being that associated with planning movements. The basal forebrain    contains nuclei that are important in learning and memory. The limbic cortex    is the region of the cerebral cortex that is part of the limbic system    , a collection of structures involved in emotion, memory, and behavior.

Cerebral cortex

The cerebrum is covered by a continuous layer of gray matter that wraps around either side of the forebrain—the cerebral cortex. This thin, extensive region of wrinkled gray matter is responsible for the higher functions of the nervous system. A gyrus    (plural = gyri) is the ridge of one of those wrinkles, and a sulcus    (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the cerebral cortex.

The head is limited by the size of the birth canal, and the brain must fit inside the cranial cavity of the skull. Extensive folding in the cerebral cortex enables more gray matter to fit into this limited space. If the gray matter of the cortex were peeled off of the cerebrum and laid out flat, its surface area would be roughly equal to one square meter.